1. Field
One embodiment of the present invention relates to an illumination optical apparatus, exposure apparatus, and exposure method and, more particularly, to an illumination optical apparatus suitably applicable to exposure apparatus for manufacturing micro devices, such as semiconductor devices, image pickup devices, liquid-crystal display devices, and thin-film magnetic heads, for example, by lithography.
2. Description of the Related Art
In the typical exposure apparatus of this type, a light beam emitted from a light source is guided through a fly's eye lens as an optical integrator to form a secondary light source as a substantial surface illuminant consisting of a lot of light sources. Light beams from the secondary light source are guided through an aperture stop disposed in the vicinity of the rear focal plane of the fly's eye lens, to be limited thereby, and then enter a condenser lens.
The light beams condensed by the condenser lens illuminate a mask with a predetermined pattern therein, in a superposed manner. Light transmitted by the pattern of the mask travels through a projection optical system to be focused on a wafer. In this manner the mask pattern is projected (or transferred) onto the wafer to effect exposure thereof. The pattern formed in the mask is of high integration and a uniform illuminance distribution must be formed on the wafer in order to accurately transfer the microscopic pattern onto the wafer.
For highly accurately transferring the microscopic pattern onto the wafer, attention is being focused on the technology of forming the secondary light source of circular shape on the rear focal plane of the fly's eye lens and changing the size thereof to vary the coherency σ of illumination (σ value=diameter of an aperture stop/pupil diameter of a projection optical system, or σ value=exit-side numerical aperture of an illumination optical system/entrance-side numerical aperture of the projection optical system). Furthermore, attention is also focused on the technology of forming the secondary light source of annular shape or quadrupole shape on the rear focal plane of the fly's eye lens to improve the depth of focus and resolving power of the projection optical system.
The conventional exposure apparatus as described above were arranged to convert an incident light beam into a light beam of a desired sectional shape by a diffractive optical element and to guide it to the entrance surface of the fly's eye lens to accomplish normal circular illumination based on the circular secondary light source, or to accomplish modified or off-axis illumination (annular illumination or quadrupole illumination) based on the secondary light source of an annular shape or quadrupole shape. The diffractive optical element herein is so designed substantially as not to generate the zero-order light (direct light).
It is, however, difficult to avoid occurrence of zero-order light from the diffractive optical element, for example, because of manufacture error or the like. In this case, the zero-order light arising from the diffractive optical element becomes unwanted light as focused in the central area (portion in the vicinity of the optical axis) on the entrance surface of the fly's eye lens and, for example, during annular illumination, it forms a surface illuminant in a relatively small size but with a high luminance in the center of the annular secondary light source. As a result, the unwanted surface illuminant is formed in the center of the annular secondary light source, which results in failure in accomplishment of desired annular illumination.
The conventional technology proposes a configuration for blocking light near the optical axis (or the zero-order light from the diffractive optical element) in the vicinity of the entrance surface or the exit surface of the fly's eye lens, in order to accomplish the desired annular illumination.
As described above, the conventional technology adopts the center blocking in the vicinity of the entrance surface or the exit surface of the fly's eye lens to avoid the influence of the zero-order light from the diffractive optical element, thereby achieving the desired annular illumination. In the circular illumination, however, a relatively small dark part is formed in the center of the secondary light source because of influence of the foregoing center blocking and thus the secondary light source of annular shape is formed instead of the desired circular secondary light source.
In the case of circular illumination using a relatively large, circular, secondary light source, i.e., in the case of large-σ circular illumination, the outside diameter of the secondary light source is much larger than that of the dark part, so that almost desired large-σ circular illumination can be accomplished with little influence of the dark part. In the case of small-σ circular illumination using a relatively small, circular, secondary light source, however, an annular secondary light source substantially different from the desired circular shape is formed because of influence of the dark part, thus failing to accomplish the desired small-σ circular illumination.
An object of the present invention is to provide an illumination optical apparatus capable of accomplishment desired annular illumination, circular illumination, and so on, without substantial influence of the zero-order light from a diffractive optical element. Another object of the invention is to provide an exposure apparatus and exposure method capable of performing good exposure under an appropriate illumination condition realized according to a pattern characteristic of a mask, using an illumination optical apparatus achieving desired annular illumination, circular illumination, and so on without substantial influence of the zero-order light from a diffractive optical element.